- Title
- Porous carbon materials from Victorian brown coal: synthesis and applications in li-ion batteries
- Creator
- Wang, Rou
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Australia owns vast Victorian brown coal (VBC) reserves, which are estimated at more than 430 billion tonnes and comprise ~20% of worldwide reserves of low-rank coal. The favourable geological locations and coal seam conditions enable economic open-cut mining. However, the high moisture content of VBC causes significant energy loss and low thermal efficiency of combustion. Therefore, more value-added and efficient applications should be considered for expanding the alternative utilisation of VBC resources. With the ever-increasing demand for energy, environmental issues become urgent due to the combustion of fossil fuels. Rechargeable lithium-ion batteries (LIBs) with high energy and power density are one of the most promising candidates for use in portable devices, electric vehicles, and smart grids. Recent research has focused on developing advanced materials for LIB anodes with high performance to replace commercial graphite, which has a limited theoretical capacity of 372 mAh/g and safety issues. Porous carbons with tunable pore structure and conductive carbon matrix have great potential for improving electrochemical performance. Low-cost VBC may be a suitable starting material for the porous carbon materials for LIBs. Its ideally low ash content, inherent porosity, and high reactivity can simplify the production process and facilitate modifications such as doping and activation, thus improving lithium-ion storage. The main objective of this project is to prepare porous carbons derived from VBC as anode materials for improving LIB performance. This study focused on the following: (1) develop different routes using facile methods for the synthesis of VBC-based porous carbons and discuss the impact of these different routes on the properties of final products, such as pore structure tailoring, carbon microstructure evolution, and heteroatoms doping; (2) investigate systematically pore structure formation using three-dimensional (3D) characterisation and the possibility of tailoring pore structure to examine the relationship between pore structure and electrochemical properties; (3) demonstrate the effect of carbon structure on the electrochemical performance of the final products; (4) explore the interaction between urea and acid-modified VBC and the subsequent impact on structural and electrochemical properties and (5) study the synergistic effect of porosity and doped heteroatoms in the porous carbons on the lithium-ion storage mechanism. To achieve these objectives, three different routes for the synthesis of VBC-based porous carbon were designed. The first route was to prepare 3D porous carbon foams (CFs) by blending VBC with coal tar pitch under pressurised pyrolysis. The pore formation under different operational conditions was investigated using micro-computed tomography (micro-CT) for a full picture of the 3D morphology. The pore structure was tailored by adjusting the operational pressure and blending ratios. The optimised carbon foam was further loaded with nickel nanoparticles as the catalyst for graphitisation followed by an air activation. The final product with a highly ordered carbon crystalline and pore structure was applied as anodes for measuring their LIB performance. After air activation, porous graphitic carbon foams exhibited a higher capacity of 463 mAh/g at a current density of 0.1 A/g. After 1000 cycles of charge and discharge under 1 A/g, the retention remained at 72% with a promising coulombic efficiency of 99.99%. The second method was to fabricate N-doped carbon nanosheets (NCNSs) from acid-modified VBC using one-pot synthesis with urea. The influence of the urea blending ratio and acid pretreatment on the mechanism of nanosheet formation was examined. Various analytical techniques were used to understand the mechanism of N doping and the changes in the physicochemical structure of VBC. Further electrochemical measurement using the obtained NCNS was performed. The NCNS sample at a 1:3 ratio of acid-modified VBC to urea displayed the highest content of pyridinic N and pyrrolic N, and delivered a reversible capacity of 513 mAh/g under 0.1 A/g. After 1000 cycles, a reversible capacity remained at 303 mAh/g at 1 A/g (capacity retention of 97%), which suggested good stability of the VBC-based NCNSs. The third method involved the synthesis of N-doped highly porous carbons (NHPCs) from VBC via direct activation by KOH and urea. The synergistic effect of hierarchically porous structure and N-doping on the electrochemical performance was examined. In favour of such designed architecture, the NHPC synthesised at a VBC-urea mixture to KOH ratio of 1:2 reached the highest specific surface area of 687 m2/g and exhibited a superior capacity of 605mAh/g at 0.1 A/g and a high-rate performance of 245 mAh/g at 3 A/g. The cycling performance at 1 A/g illustrates a reversible capacity of 268 mAh/g after 1000 cycles with high stability. The retention remained at 88% after cycling. This thesis discussed the effects of different synthesis routes on the structural properties of porous carbon materials synthesised from VBC, including carbon foams, N-doped carbon nanosheets, and N-doped highly porous carbons. The introduction of N heteroatoms improved the electrical conductivity of the carbon matrix. Benefits were achieved from the synergy effect of high porosity and N-doping in terms of improved stability, reversibility and high rate capacity. The work presented here shows the potential of VBC for the large-scale synthesis of porous carbons as LIB anodes via a facile and low-cost technique.
- Subject
- Victorian brown coal; LIBs; carbon anodes
- Identifier
- http://hdl.handle.net/1959.13/1504552
- Identifier
- uon:55551
- Rights
- Copyright 2022 Rou Wang
- Language
- eng
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| View Details Download | ATTACHMENT01 | Thesis | 84 MB | Adobe Acrobat PDF | View Details Download | ||
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